1. Field of the Invention
The present invention relates to sprayable polyurethane compositions and to methods for coating surfaces with such compositions. In particular, the polyurethane coating of the invention is low density, durable, and has a thickness greater than that achievable with previous polyurethane coatings.
2. Description of the Related Art
A durable, low density, thick, high build rate coating with low volatile organic compound (VOC) emissions has been desired as a protective coating for many years. Attempts at producing a coating having those characteristics has resulted in compromising one or several of the desired properties. When low density coatings are fabricated they tend to have low toughness or abrasion resistance. Furthermore, most low density coatings require a significant amount of organic solvent to carry the coating during spraying applications, which contributes significantly to the problem of VOC emissions. Thick coatings tend to be spongy or brittle and applying that type of coating to a vertical surface is a slow and tedious process due to slumping problems or curing/out gassing times required. The majority of low VOC coatings available at the present time are only acceptable for thin coating applications (less than 20 mils dry film thickness), or have an exceedingly long dry time between applications which prohibits the successful fabrication of high build rate and/or thick coatings.
These and other material and process problems associated with thick, durable, low density coatings necessitated the development of a low VOC coating that could be applied by a conventional spraying method.